1. Field of the Invention
The present invention relates to a hydraulic control system for a working machine such as a hydraulic excavator.
2. Description of the Related Art
According to a conventional hydraulic system in a working machine, when a large load is imposed on a hydraulic actuator with a consequent increase in internal pressure of the hydraulic system, working oil is released to a tank through a relief valve to prevent an excessive increase of pressure and thereby prevent a damage to hydraulic devices.
In this case, a large proportion of working oil fed from a hydraulic pump during operation of the relief valve is released directly to a tank through the relief valve without being fed to the hydraulic actuator, so that a large proportion of pump power is consumed as a loss of the relief valve, thus giving a rise to the problem that the energy efficiency is deteriorated.
For example, FIGS. 13A to 13D are explanatory diagrams showing a pressure-flow rate response relation observed when positive control alone is performed. As shown in FIG. 13A, when pilot pressure PI fed from a remote control valve to a control valve is fed stepwise up to FULL by operating an operating lever, a pump flow rate Qp also becomes a maximum flow rate stepwise as in FIG. 13C. However, for example in the case where a hydraulic actuator is a swing motor, the speed of the swing motor does not increase rapidly under the influence of inertia of a swing body. Consequently, an incoming flow rate into a supply-side pipe in the swing motor becomes larger than an outgoing flow rate from the discharge side pipe in the same motor, resulting in that, as shown in FIG. 13B, the internal pressure of the supply pipe, i.e., pump pressure Pp, rises up to relief pressure Pr and, as shown in FIG. 13D, a surplus flow rate of working oil is discharged as relief flow rate Qr from a relief valve.
In Japanese Patent Laid Open No. Hei 8-219104 (Patent Document 1), the positive control is performed so that the higher a pump discharge pressure, the smaller the pump discharge quantity. In Japanese Patent Laid Open No. Hei 10-246204 (Patent Document 2), under a negative control, a throttle is installed on the downstream side of a relief valve, and when the pressure on an upstream side of the throttle increases with an increase of the relief quantity, the pump flow rate is decreased to lower the relief flow rate. Further, in Japanese Patent Laid Open No. 2002-038536 (Patent Document 3), under the negative control, temperatures of working oil on both upstream and downstream sides of the relief valve are detected and when the difference between the temperatures has become large, the pump flow rate is decreased to decrease the relief flow rate.
However, according to the technique disclosed in Patent Document 1 it is impossible to control the pump discharge quantity so as to decrease by an amount matching the relief flow rate. Therefore, if the decrease quantity of the pump discharge quantity is smaller than the amount matching the relief flow rate, the pump discharge pressure exceeds the relief pressure, causing operation of the relief valve, with consequent creation of a large relief flow rate posing the problem that the energy efficiency is deteriorated. If the decrease quantity of the pump discharge pressure is larger than the amount matching the relief flow rate, the relief valve fails to operate and there also arises the problem that the accelerating ability is impaired due to an excessive lowering of the pump discharge pressure.
According to the technique disclosed in Patent Document 2, there occurs a pressure loss due to the throttle provided on the downstream side of the relief valve and hence it is impossible to improve the energy efficiency. According to the technique disclosed in Patent Document 3, heat capacity of the relief valve causes a very large time lag between the creation of relief flow rate and the rise in temperature of the relief valve, so that an excessive time delay occurs from the time when relief flow is created to cut-off of pump flow rate, and thus the relief loss is not fully diminished. Even after the arrival of relief flow rate at zero, the pump flow is cut off for the detection of remaining heat and it becomes impossible to obtain a required driving force.
In case of using only a pressure feedback control instead of positive and negative controls, the control is made for example in such a manner that meter-in opening of a control valve becomes small when the lever operation amount is made small. In this case, in the pressure feedback control, the pump flow rate is not controlled in accordance with the lever operation amount and therefore the pump discharge pressure is always controlled integrally by a high pressure close to the relief pressure. As a result, a differential pressure in a meter-in circuit becomes very high, producing an extremely heavy meter-in loss, whereby the operability is deteriorated. For this reason, the adoption of only the pressure feedback control instead of the ordinary control such as the positive control has not been actually applied.